What you need are:
* your original HP-UX installation CD's (core OS + application)
* your HP certificates with codewords.
Then fire up an interactive swinstall session. Browse through the menu to find an option
add codeword. By putting a correct codeword or key you will 'unlock' hidden install packages on the mounted CD. With the right key you will get a package with description 'H-UX 16-user license'. Then you can select the package and install it.
Mind you, I don't know on which CD you should be looking for so you'll have to try them all